This invention relates to methods and apparatus for sensing alternating current signals in high voltage conductors.
In the design of high voltage current sensing and measuring apparatus, conventional devices (typically utilizing transformers) have employed magnetic coupling phenomena for sensing and measuring current. Typically, the transformers have been terminated in very low impedances, thereby necessitating the use of iron cores to increase the transformer magnetizing inductance to yield a desired degree of proportionality between transformer primary and secondary current. Disadvantages associated with the use or iron-core transformers include the possibility of saturation of the core during the occurrence of a high fault current, the possibility that the iron core may exhibit residual magnetism after the occurrence of a high fault current, and the possibility that the magnetic hysterisis of the core material may introduce inaccuracies in the current replica signal produced by the transformer.
Hall Effect devices have also been used as current sensing and measuring devices, but have proven disadvantageous due to the excessive dependence of the Hall voltage on temperature and due to the necessity to provide an accurate, stable bias current to the Hall Effect device.
These disadvantages reduce the accuracy of the current replica signal produced by such conventional current sensing and measuring apparatus. This in turn limits the usefulness of such apparatus in power systems which utilize components such as solid state fault detectors which require, as an input, replica signals closely proportional to the actual system current and voltage signals.
A further problem with which the designers of high voltage electronic current sensing and measuring apparatus have had to cope is the prevention of short circuiting of the high voltage conductor through the current sensing and measuring apparatus. The close physical proximity required to ensure adequate magnetic coupling between the primary and secondary winding of a conventional current transformer makes electrical insulation of the primary and secondary windings progressively more difficult and expensive as the voltage of the conductor increases.
An object of the present invention is to provide a new and improved method and apparatus for sensing current in high voltage conductors and, in so doing, to substantially avoid problems associated with the use of iron-core transformers or Hall Effect devices, and also to avoid the problem of short circuiting through the current sensing apparatus.
A further object of the present invention is to provide a new and improved method and apparatus for sensing a current in high voltage conductors which utilizes input power from the high voltage conductor in the performance of the method and in the operation of the apparatus.